Calcium carbonate - the problem with better quality paper

Calcium carbonate is used as a filler in the basesheet and in the paper coating as a pigment. It provides brightness and a more blue-white shade than clay does. Calcium carbonate is used in neutral or alkaline paper making, which results in a more permanent sheet than acid paper making by reducing the yellowing and brittleness of paper as it ages.Trace amounts of calcium carbonate can even be found in some ink formulations where it is used as an extender. Higher levels are typically present in magenta ink. Calcium carbonate buidup on the blanket often shows up as a hole in the center of halftone dots - especially in smaller, or highlight, dots as in the example above.
The upside in the move from acid (clay filler) to alkaline (calcium carbonate filler)
Unsurprisingly, the benefits of calcium carbonate has resulted in a move, that began in Europe, from acid paper toward alkaline paper.

Alkaline paper provide several advantages over acid paper:

• It's less polluting to the environment
• Has better permanence
• Provides improved sheet strength
• Uses fewer trees per ton of paper produced
• Has increased opacity and brightness
• Faster ink set for quicker turn around
• A more cost-effective paper manufacturing process

Today, almost all of the North American uncoated wood-free sheet capacity uses an alkaline or neutral papermaking process with calcium carbonate as a filler and pigment.

The downside
However, alkaline papers can create a whole set of printing issues for printers. Calcium compounds can leach out of the paper during the printing process. This leaching out can be exacerbated by highly acidic or overly aggressive fountain solutions especially on uncoated papers. When this happens, the calcium carbonate pigments migrate to the upper form roller. Once there, they are milled into the ink and dispersed throughout the dampening system build up and may overwhelm the printing system.

The impact of calcium carbonate leaching can include:

• Tinting on the printed sheets
• Toning on the plate
• Blanket piling and picture framing effect
• Build-up on non-image area of the plate weakening receptivity of water (scumming)
• Roller glazing
• Contamination of fountain solution and increase pH and conductivity
• On negative plates, the calcium carbonate crystals from the paper (two to three microns in diameter) may accumulate on small dots and cause blinding.

Calcium carbonate issues are most often experienced in high volume web printing with uncoated paper where calcium carbonate is used as a relatively unsealed basestock filler.

Symptoms of calcium carbonate contamination may include:

• Progressively poor ink transfer usually seen as dot sharpening
• Ink roller stripping
• Fountain solution progressively becoming more alkaline (if it's not buffered for alkalinity).
• High conductivity gain of fountain solution
• Excessive foaming of fountain solution.
• Build-up of calcium on the ink rollers. This typically appears as a white haze which is not easily removed with conventional roller wash.
• Calcium deposition on the blanket surface (a white haze which cannot easily be removed by plain water) which interferes with the ability of the blanket to transfer ink properly and print a sharp dot with clean background.
• Build-up or piling in the non-image area of the blanket.
• Progressive toning or scumming as a result of increased alkalinity, poor water receptivity, poor ink transfer, and accelerated plate wear.
• The sizing particles attached to the calcium carbonate pigments may activate the ink driers prematurely, resulting in either plate scumming or plate blinding with blanket and roller glaze impeding the transfer of ink which in turn necessitate frequent, but ineffective, wash-ups.
• Problems specifically with magenta or red pigmented inks.